Misunderstanding

People who don't follow the later parts of History's Strongest Disciple Kenichi manga (the image was taken from chapter 301) probably won't understand this joke.

To put a long story short the two young men, Kenichi and Boris, are actually rivals and are fighting one another. The lady teacher however misunderstood the situation and thought that they are playing in the water as good friends. She also feels happy for Boris as she thought that he had finally found a friend.

Personally though, I recommend reading the manga series instead. It's an insane, awesome and hilariously funny series.